For large solar projects, iMaps and pvPlanner data are suitable for use in the pre-feasibility phase. In the due diligence phase we strongly recommend use of time series or TMY data. The following reasons apply:
SolarGIS time series or TMY datasets are prepared using the latest SolarGIS model and are more accurate when compared to data from iMaps or pvPlanner. Data in iMaps and pvPlanner are pre-calculated, and for some regions were calculated using older version of SolarGIS model and input data.
PVsyst uses 8760 hourly averages for simulation of energy yield. When monthly averages are imported a synthetic TMY dataset with 8760 hourly values is generated. The synthetic TMY may not give a realistic representation of the climate of project site. Moreover, coherence of irradiation and temperature values is lost. Deviation in PV yield output when using monthly average instead of time series can be as high as ±2% for annual values and ±7% for monthly values. In some complex climates, higher deviations can be observed.
Calculation of inter-annual variability, required for calculating one-year P90 energy estimates, is possible only when multi-year time series data is available.